This invention relates to an improvement of a data backup system, in particular, a backup system suitable for a database.
In recent years, database backup systems of the type that uses two computer systems have come to be employed widely. Backing up a database is to make a copy of data in one of two computer systems and store the copy in the other computer system. The computer system of which data is backed up is called an active system (or a primary site) and the computer system to which the data is backed up is called a standby system (or a secondary site). The data stored in the standby system is called backup data. Transaction log transfer is known as a database backup method that uses a double system (see, for example, Christos A. Polyzois, Hector Garcia-Molina, Evaluation of remote backup algorithms for transaction-processing systems, ACM Transactions on Database Systems (TODS), vol. 19, No. 3, p. 423 to 449, September 1994).
Transaction log transfer is a method of transferring a data update record (a log) of the active system to the standby system to create backup data in the standby system. Such the transaction log transfer makes sure that backup data in the standby system is the same as data in the active system at the start of backup operation. After the backup operation is started, the data update record is transferred to the standby system. In the standby system, backup data is updated based on the transferred data update record, and thus creates the latest backup data.
A computer system in general contains a storage system in which data of the computer system is stored. Some known storage systems have a function of copying data between two storage systems (remote copy) (see JP 2004-78746 A). The remote copy function is to copy data of one storage system to the other storage system by transferring the data via a network.
Recent storage systems increasingly employ disaster recovery systems that use remote copy. A disaster recovery system (hereinafter referred to as “DR system”) remote-copies data from a site that is in operation (primary site) to a remote site (secondary site) for the purpose of enabling the business to continue after a failure caused by a natural disaster or the like.
The DR system in some cases places the secondary site several hundreds km or more away from the primary site. In such the cases, the construction and maintenance of a network that connects the secondary site to the primary site cost more. The maintenance cost is particularly high when a wide-band network (of, e.g., a few Gbps) is employed as a several hundred-km long network that connects the two sites. To cut the cost of the DR system, the network between the two sites has to be built from a narrow-band network (of, e.g., 100 Mbps). A known DR system that accommodates this condition is a log-based DR system which copies a database of the primary site by transferring only logs from the database of the primary site to the secondary site and carrying out log application in the secondary site. In the log-based DR system, a computer of the secondary site reads a log transferred through remote copy and a log application control unit applies the read log to a database of the secondary site. With communications between the primary site and the secondary site limited to transfer of logs, the log-based DR system can employ a narrow-band network for the network that connects the two sites.